High power fiber gain media system achieved through power scaling via multiplexing

ABSTRACT

Power scaling by multiplexing multiple fiber gain sources with different wavelengths, pulsing or polarization modes of operation is achieved through multiplex combining of the multiple fiber gain sources to provide high power outputs, such as ranging from tens of watts to hundreds of watts, provided on a single mode or multimode fiber.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuing application of provisional application,Serial No. 60/028,604, filed Oct. 22, 1996, pending, which isincorporated herein by reference thereto.

FIELD OF THE INVENTION

This invention relates generally to scaling power of fiber light sourcesfor coupling high optical output power to optical devices andapplications and, more particularly, to high power fiber gain mediasystem having tens to hundreds of watts, such as 50 and 100 watts ormore, of output power achieved through WDM, TDM and/or PBM combining ofmultiple semiconductor gain medium sources or multiple fiber gain mediumsources.

BACKGROUND OF THE INVENTION

Due to the development of reliable high power laser diodes and diodearrays, it is now possible to achieve higher power from all types ofsolid state lasers. Typical solid state lasers, such as Nd:YAG lasers,typically operate over a fairly narrow wavelength determined by a narrowband of atomic transitions. They are also limited in their temporaloperation, e.g., their pulse modulation is limited. On the other hand,fiber gain media, such as rare earth doped fiber gain sources, can beoperated comparatively over a wide wavelength band. As an example, Ybdoped fiber sources are operative over a wavelength range of about 1060nm to 1150 nm depending on a number of design parameters including fiberlength and the application of wavelength-selective feedback. Also,because of the high gain of the fibers, they may be operated asamplifiers providing precise control over the temporal output of thelaser source. Thus, rare-earth doped fiber gain sources, versus pumpedsolid state lasers, may be controlled in their temporal output over awide range of pulse lengths and modulation rates.

The use of a single mode fiber for linear power scaling, i.e., toincrease or up-scale the optical power, is better because of forcedlaser oscillation in single transverse mode. Also, fiber lasers offer alow cost, easily produce power source at selected wavelength operationfor telecommunications, printing, signal detection and medicalapplications. The upper limits of power scaling in conventional singleclad, rare earth doped monomode fibers is limited because of thenumerical aperture (NA) and core size incompatibility of these singlemode fibers with the beam parameters and NA of high power laser diodesand laser diode arrays. As outlined in the paper of H. Zellmer at al.,entitled, “High-Power CW Neodymium Doped Fiber Laser Operating at 9.2 WWith High Beam Quality”, OPTICS LETTERS, Vol. 20(6), pp. 578-580, Mar.15, 1995, to scale the pump power, a larger fiber core diameter that isadapted to the emitter dimensions of the high power laser diode or diodearray. However, reduced beam quality results because an increase fiberdiameter permits multimode operation.

To overcome this problem, specially configured double clad fibers havebeen developed where the pump radiation is launched directly into amultimode waveguide having an inner cladding surrounding a single modecore, i.e., a pump core or inner cladding which has a larger NA andlarge cross area which is compatible with the beam parameters of highpower laser diodes or arrays. A double clad fiber, for example,comprises a single mode core, doped with a rare earth such as Yb, Nd, Eror other rare earth dopants, or combination of such dopants, such as,Er:Yb, surrounded by an inner cladding of lower refractive indexmaterial compared to the core. High output power of the fiber gainsource is achieved by launching multimode pump light into the pump coreof the fiber having a wavelength corresponding to the pump absorptionbandwidth of the rare earth dopant in the fiber core. The pump lightpropagates in the multimode inner cladding and is absorbed into theactive monomode core over the length of the fiber. The multimode innercladding permits the multi-traversing of the core by the light with awavelength corresponding to excited emission state of the doping atomsin the pump core for bring about stimulated transition of the excitedatoms to a lower energy level resulting in gain for signalamplification. As a result, multimode pump light from a high power diodelaser array is converted into single transverse mode power output ofseveral watts from a single mode core of the double clad fiber. Forexample, for a typical 100 μm by 300 μm multimode pump light with a 100μm by 300 μm diameter beam and a 0.47 NA, the beam may be efficientlycoupled to the inner pump cladding of the fiber. The output from thefiber is a single mode beam with a 10 μm diameter and a 0.1 NA. This isabout a three order increase in coupled brightness.

These double clad fiber gain sources can be operated as fiber amplifiersor fiber lasers. The laser configuration employs feedback means such asin the form of a pair of reflectors or fiber Bragg gratings making it arelatively simple structure, but is limited in most cases to cwoperation. The amplifier configuration has the advantage of accuratecontrol of the temporal output of the fiber source. The output opticalpower of either the laser or amplifier configuration is limited by theamount of pump light that may be injected into the fiber, theoptical-to-optical conversion efficiency, and the maximum powerachievable before the onset of fiber degradation. For a given rare earthdopant, the theoretical conversion efficiency is around 40% to 70% and,in actual practice, similar levels of conversion efficiency have beenachieved.

The output power from a single fiber gain source can be increased bypumping the fiber source from both ends or at multiple points along thelength of the fiber. Also, the output power from a single fiber gainsource can be increased by increasing the size of the fiber and itsnumerical aperture (NA). In practice, however, the size of the fiber islimited to a diameter of several 100 μm and its NA is approximately0.45. The NA is limited by availability of suitable polymers that can beemployed for the outer cladding of the fiber. Of course, the outputpower of the fiber gain source with a given input aperture and NA can beincreased by increasing the output power and brightness of the pumpsource for pumping the fiber medium. Typically, this can be accomplishedby the use of multimode and multiemitter semiconductor laser diodearrays or laser bars as pumping sources. The format and brightness ofthe array or bar should be optimally matched to the etendue of the innercladding of the fiber. Theoretically, the brightness of the sourceshould not be detrimentally affected in accomplishing this reformattingbut, in practice, the brightness is significantly lower. As an example,a typical fiber coupled laser bar providing 17 W of cw power may becoupled to a fiber pump core having 170 μm by 330 μm rectangularcross-section aperture and an NA of 0.45.

It is known to scale power in a fiber source by injecting pump light inboth ends of the fiber source, such as exemplified in the to Huber U.S.Pat. No. 5,268,910. Also, it is known to scale to even higher outputpowers in fiber sources by increasing either the pump power, such ashigher power semiconductor pump sources or using multiple semiconductorpump sources, or by increasing the pump efficiency, such as bydecentering the active core of a double clad fiber relative to thesurrounding pump (inner cladding) core or use longer fibers withperiodic fiber bends to convert, in both of these cases, more of themultimodes in the pump core. See, for example, H. Zellmer at al., supra;the patent to Chirravuri et al. U.S. Pat. No. 5,287,216; and the toDelavaux U.S. Pat. No. 5,185,826. Moreover, it has been previouslydisclosed in work published by Lew Goldberg at al. to scale output powerby connecting in series a plurality of fiber gain source stages, such asdouble clad fiber amplifiers. In this case, multiple fiber sources arecoupled in series, and the power from the first fiber source is coupledinto the second fiber source and so. Each fiber source may be pumpedfrom one or both ends, such as through the employment of dichroicmirrors which separate between pump light, which is typically around 808nm or 915 nm, and the output wavelength, which typically around 1.06 μmor 1.55 μm. However, there are two disadvantages in this type of scalepower system. First, the power levels in cores of fiber sourcesdown-line in the system may become so high causing fiber coredegradation. Second, the level of coupling losses between fiber gainsource stages will limit the number of stages that can be effectivelycoupled together. For example, assuming 20% coupling losses betweenmultiple source stages and 10 watt single stage fiber sources, a netpower gain cannot be increased above 50 watts of output power becausethe coupling loss will equal the power achieved in any subsequentlycoupled source stages. Of course, improvements in coupling efficiencybetween stages will result in higher power levels, but there arepresently limits on how high of a coupling efficiency will be reasonablyachieved in stage coupling.

Another possible approach for scaling power in serially connected fibergain source stages is side pumping each fiber source periodically alongthe length of the stage fiber. This technique is illustrated for singlemode fibers in the patent to5 Whitley et al.

A further approach for scaling power is employing polarization beammultiplexing (PBM) to combine a plurality of beams into a single outputhaving different polarization modes as long as they are orthogonallypolarized. This principle is known in the art, such as evident from thepatent to Pantell et al. U.S. Pat. No. 5,311,525 wherein in column 14,there is a discussion of optical devices for coupling of optical energyor radiation between two different sets of polarization modes.

A still further approach for increasing power to provide a high powerfiber amplifier is the utilization of a plurality of pump double cladfiber lasers operating at pump wavelengths within the pump band of thefiber amplifier as discussed in the to Huber patent U.S. Pat. No.5,187,760 and disclosed in FIGS. 7 and 10 and in column 8, lines 29-42and lines 48-54.

However, there is a need to enhance scaling of power via multiple fibermedium sources of different wavelengths and how this can be accomplishedwithin the limited gain band of such sources, which is the subject ofthis application.

While improvements in output power can be achieved by improving theoptical coupling and formatting between the laser diode array or barsources and the input of the optical fiber media or by providing higherbrightness of such laser diode pump sources or a combination thereof, itwould also be advantageous to further improve the power scaling of thesefiber gain medium sources, particularly double clad fiber sources,employing utilizing existing semiconductor pump sources and fibercoupling technology.

It is an object of this invention to provide optical gain media systemsthat provides scaling to high optical output powers, such as severaltens to hundreds of watts of output power.

It is another object of this invention to produce optical power scalingsystem through wavelength division multiplexing (WDM), time domainmultiplexing (TDM) and/or polarization beam multiplexing (PBM).

It is a further object of this invention to provide a power scalingthrough multiplex combining of multiple rare-earth fiber gain sources bytaking advantage of their wide wavelength band and temporal operation.

SUMMARY OF THE INVENTION

According to this invention, high optical outputs can be achievedthrough improved power scaling by multiplexing multiple fiber gainsources with different wavelengths, pulsing or polarization of operationthrough multiplex combining. Fiber gain sources with wide pumpwavelength absorption bandwidth can be produced as multiple pump sourcesand their outputs combined through a WDM, TDM or PBM component toproduce a single output on a single fiber capable of handling high powerlevels.

In a first feature of this invention, pumped semiconductor or fiber gainsources with different wavelengths of operation are enhanced in thenumber of added pump sources through the fruition of differentwavelength fiber gratings, one relative to each pump source providing aplurality of pump lights wavelength all within the gain bandwidth of thesemiconductor gain sources or within the absorption bandwidth of therare earth dopant or dopants employed in the pumped fiber sources. Theoutputs of the multiple wavelength sources are then combined employingwavelength division multiplexing (WDM) producing a high power outputbeam within tens to hundreds of watts of power. Further, enhanced WDMcoupling can be achieved with the use of fused taper couplers. Couplingwith these types of couplers permits efficient coupling of light ofdifferent wavelengths and can be adapted to couple light within about±10% of peak transmission wavelengths in pairs of coupled fiber lasersources with paired outputs of coupled pairs coupled via subsequentfused taper couplers adapted to have peak transmission wavelength bands(±10%) of previously combined wavelength outputs from upstream orprevious coupled pairs of fiber laser sources. These high power sourceshave high adaptability and acceptance for use in applications wherewavelength(s) of output is of less importance, such as in printingsystems, such as thermal or xerographic printing, or in materialprocessing involving thermal treatment or marking, such as metal cuttingand drilling, or marking such as intelligence (e.g. alpha-numeric orgraphic information) on or in a surface, or in surgery such as in tissueremoval. In the case where such power sources are employed inamplification of telecommunication signals, such as 1.3 μm or 1.55 μmcommunication systems, the power output of these pumping systemseliminates the need for frequent repeaters. Moreover, pumping can beaccomplished at opposite ends of the fiber amplifier, i.e., counterpropagating WDM coupled pump sources, to increase amplification andusing in-line wavelength filters to isolate the counter-propagating pumpfiber laser sources from one another.

In a second feature of this invention, the gain spectra of fiber laserpump sources can be extended with the combination of such fiber dopedsources with Raman fiber lasers to provide Raman wavelength shifting towavelengths beyond the wavelength spectra available from the fiber dopedsources.

In a third feature of this invention, pumped fiber gain sources withdifferent polarization propagation modes can be utilized to enhance thenumber of fiber laser sources that can be combined with the polarizationbeam multiplexing (PBM) of the outputs comprising different polarizationpropagation modes in combination with the wavelength divisionmultiplexing (WDM) with pumped fiber sources with different wavelengthsof operation, their ultimate combination producing a high power outputbeam within tens to hundreds of watts of output power.

In a fourth feature of this invention, fiber laser or amplifier sourcesare operated in a pulse mode so that the outputs of the fiber lasersources can be time domain multiplexed (TDM) producing either a cw orpulsed high power output beam within tens to hundreds of watts of outputpower.

In a fifth feature of this invention, first and second sets or groups offiber laser sources are doped with different active dopants havingoverlapping or adjacent gain spectra so that the overall gain spectra ofpossible fiber laser sources of different and separated wavelengthsprovided for WDM combining is increased thereby producing a high poweroutput beam within tens to hundreds of watts of output power.

In a sixth feature of this invention, a fiber gain source comprises twoconcentric, monomode gain cores surrounded by an outer pump core whichmay have a circular or rectangular cross-section or othercross-sectional configuration, the first of the pumped coresincorporated with a first active dopant and the second of the pump coresincorporated with a second active dopant. Pump light is supplied to thepump core having a wavelength band within the absorption band of boththe first and second active dopants. Such fiber gain sources with widepump wavelength absorption bandwidth permit pumping of a single fiberwith multiple pump sources combined through a WDM component. Such WDMcombined fiber gain sources produce a high power output beam within tensto hundreds of watts.

In the different embodiments of this invention, the multiplex couplingand the ultimate combined output to a single optical fiber, such as amonomode fiber, may be accomplished using pure, undoped silica corefibers, which may have fluorides in their claddings for waveguiding, towithstand the high power levels combined from so many fiber amplifier orlaser sources. A relatively short output fiber should be used to preventfiber nonlinearity including Raman scattering and four wave mixingeffect.

An important attribute relative to the first feature of this inventionis the use of precise fiber gratings to increase the number of possiblefiber sources that may be WDM combined. Also, such gratings may bechirped to reduce fiber laser noise but has a grating period stillsufficiently narrow to permit wavelength selection and WDM combining.Also, WDM combining can be accomplished by the use of dichroic mirrorsor grating mirrors as well as fused taper couplers. Further, quickcoupling means may be employed between the fiber source and its outputfiber grating for producing a prescribed output wavelength so that anydefective or damaged pumped fiber laser source may be removed from thefiber laser system while leaving their output fiber grating intact inthe system. Thus, single source replacement is accomplished withoutreplacement of the entire system and without concern of outputwavelength maintenance and its subsequent WDM combining since thatfunction remains governed by the intact output fiber grating.

Other objects and attainments together with a fuller understanding ofthe invention will become apparent and appreciated by referring to thefollowing description and claims taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an embodiment for power scaling of thisinvention utilizing the bandwidth of multiple fiber laser sources forachieving different output wavelengths that are WDM combined.

FIG. 2 is refractive index cross sectional representation of a monomodefiber preferred for high power transmission in the hundreds of watts toseveral kilowatts range.

FIG. 3 is a schematic view of a modified form of the embodiment shown inFIG. 1 for power scaling of this invention where a plurality of pumpsources are WDM combined to produce a single output to pump a fiberamplifier.

FIG. 4 is a schematic view of an extended form of the application shownin FIG. 3 where sets of a plurality of pump sources are WDM combined topump respective fiber amplifiers which are WDM combined to produce asingle output.

FIG. 5 is a schematic view of a basic embodiment for WDM combining ofmultiple fiber laser sources for power scaling utilizing the bandwidthof multiple fiber laser sources via frequency selecting fiber gratings.

FIG. 6 is a first detailed example of an embodiment of this inventionrelative to the use of fused couplers as WDM devices.

FIG. 7 is a second detailed example of an embodiment of this inventionrelative to the use of fused couplers as WDM devices.

FIG. 8 is a detailed example of an embodiment of this invention forpower scaling of this in,mention utilizing the bandwidth of multiplefiber laser sources.

FIG. 9 is a schematic view of a WDM combiner in the form of a fusedbiconical coupler for use with this invention that accepts a limitedbandwidth of wavelengths from multiple laser sources.

FIG. 10 is schematic view of a quick disconnect scheme that may beutilized in connection with any of the embodiments of this invention.

FIG. 11 is a schematic view of another embodiment for power scaling ofthis invention utilizing the bandwidth of multiple fiber laser sourcesin combination with chirped gratings.

FIG. 12 a schematic view of a WDM combiner in the form of a gratingreflector.

FIG. 13 is a schematic view of another embodiment for power scaling ofthis invention utilizing groups of fiber lasers doped with differentactive dopants that together provide an extended gain spectra andprovision for additional generated wavelengths that may be WDM combined.

FIG. 14 is an illustration of the gain spectra for the fiber lasergroups of FIG. 13.

FIG. 15 is a schematic view of a sixth embodiment for power scaling ofthis invention utilizing a novel dual core pumped fiber laser accordingto this invention.

FIG. 16 is a cross sectional view of the dual pumped core fiber lasershown in FIG. 15 taken along the line 16—16 in that figure.

FIG. 17 is a cross sectional view of the single pumped core fiber lasershown in FIG. 15 taken along the line 17—17 in that figure.

FIG. 18 is a schematic view of a further embodiment for power scaling ofthis invention utilizing time division multiplexing (TDM) of multiplefiber amplifiers.

FIGS. 19A and 19B are graphic illustrations of the utilizing thestructure shown in FIG. 18 with time division multiplexing of multiplefiber amplifiers to provide a pulse output.

FIG. 20 is a graphic illustration of a further embodiment for powerscaling of this invention utilizing the structure shown in FIG. 17 withtime division multiplexing of multiple fiber amplifiers to provide apulse output.

FIG. 21 is a schematic view of another embodiment for power scaling ofthis invention utilizing both the different wavelengths and differentpolarization modes of multiple fiber laser sources that are WDM and PBMcombined.

FIG. 22 is a schematic view of a still further embodiment for powerscaling of this invention utilizing, in addition, Raman pumped fiberlasers according to this invention.

FIG. 23 is a wavelength transmission band diagram for a Yb doped fiberlaser with an illustration of Raman shifting of wavelength outputs forembodiment of FIG. 22.

FIG. 24 is an illustration of applications for the different embodimentsof the fiber laser systems according to this invention.

FIG. 25 is a schematic illustration of another embodiment of thisinvention where the fiber gratings employed to stabilize the output ofplural laser sources with the WDM device positioned between the sourceoutputs to be combined and the fiber gratings formed in the output fromthe WDM device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The following examples of embodiments and their discussion involvingparticular pumping sources in different configurations, different gainmedia, and particular wavelengths or polarization modes, and particularfiber dopants with specified gain spectra are intended to be onlyexemplary, as many different combination of these components can bevisualized as is recognized by those skilled in this art. Also,different types of optical couplers may be employed in any oneembodiment, where feasible, such as dichoric mirrors and directionalcouplers, such as fused fiber couplers, n×n fused fiber couplers,polished couplers and D-fiber couplers.

Reference is now made to FIG. 1 which illustrates a first embodiment ofthis invention for scaling the output power of fiber gain sources, inparticular double clad fiber sources, such as double clad fiberamplifiers and lasers, through the employment of the available bandwidthof these fiber gain sources. As shown in FIG. 1, a power scaling fiberlaser system 10 comprises a plurality of double clad fiber lasers 14,L₁, L₂ . . . L_(n) having optical feedback means in the form of cavityend mirrors 13 and output transmission fiber gratings 16, G₁, G₂ . . .G_(n). As is well known in the art, other forms of feedback mechanismsand their combinations may be employed to establish a resonant lasingcavity in each of the fiber lasers 14. For example, mirrors 13 may beused at both end of fiber lasers 14 with the output mirror coated with awavelength filter to pass light of a wavelength different from that ofadjacent fiber lasers. However, in improving the scaling of powercapabilities, it is preferred that a fiber grating be employed toprovide for selective wavelength separation into a larger number ofpossible different outputs for a given gain spectra of a chosen activedopant utilized for the plural fiber laser sources 14 in system 10.

Fiber lasers 14 may be single mode although multimode or double cladfibers are preferred in order to achieve higher output power. Fiberlasers 14 are provided with an active dopant in their pumped monomodecores, such as rare earths of neodymium (Nd³⁺), ytterbium (Yb³⁺), erbium(Er³⁺), thulium (Tm³⁺), or holmium (Ho³⁺) doped fiber cores or codopedcombinations thereof such as a combination of erbium and ytterbium(Er³⁺:Yb³⁺) doped core, with a pump core or inner cladding that hascircular or noncircular symmetry, such as rectangular geometry. Yb dopedfiber lasers are preferred over Nd doped fiber lasers because of thelarger gain spectrum of Yb material which is homogeneously broadened, orYb material as combined with other rare earth materials, such as Er:Yb.Thus, other preferred choices are Er doped fiber lasers and Er:Yb dopedfiber lasers. Different fiber lasers 14 may be doped with different rareearth dopants as long as the combined outputs of each of the lasers areof all different wavelengths.

Pump sources 12, P₁, P₂ . . . P_(n) provide pump light input to each offiber lasers 14 and may be comprised of one or more high power,semiconductor laser sources, such as a high power single emitter laserdiode or a multi-emitter laser array. The output wavelength of the pumpsources are selected as a function of the absorption band of the activedopant of the fiber lasers 14. Also, pump sources may be comprised ofsuch a high power, semiconductor laser sources providing pump input to afiber laser which, in turn, is provided as the pump input to arespective fiber laser 14 of system 10. In any of these cases, the pumpradiation is launched directly into the pump core or inner cladding ofthe double clad fiber laser 14.

A unique but critical aspect of power scaling system 10 is that theoutput wavelength of each fiber laser 14 is controlled by a fiber Bragggrating 16 at its output, formed in the fiber by conventionalultraviolet light phase masking or plural beam interference techniques.Gratings 16 to provide a designated wavelength selection from therespective fiber lasers within the absorption bandwidth of theparticular rare earth dopant provided in the fiber core of fiber lasers14. The use of fiber gratings 16 is an important aspect of thisinvention because the desired reflected and resulting resonant cavityand transmission wavelength established can be accurately determined to0.1 nm employing the ultraviolet phase masking or plural beaminterference process for creating the grating periodicity that iscapable of being used on mass production basis. The suggestion of fiberlasers as wavelength multiplexed pump sources for amplifiers has beendisclosed in the past, as mentioned in the Background with regard toHuber patent U.S. Pat. No. 5,187,760. However, in this reference, thefiber laser pump sources are not stabilized in wavelength at theiroutput with an in-line fiber Bragg grating, such as is the case here sothat, as a consequence, it is not possible to provide a plurality ofpump sources narrowly spaced in wavelength. Huber discloses resonantcavities with mirrors, but there is no disclosure of fiber gratings forthe purposes of wavelength stabilization. Without good wavelengthstabilization, trying to achieve a narrow wavelength spacing would beimpractical, as changes in wavelength would effect WDM combiningefficiency. Achieving a narrow wavelength spacing is of crucialimportance as the absorption cross-section of the employed activedopant, such as Er or Nd or Yb, drop off rapidly on either said of theirpeak absorption wavelength. Employing a narrower wavelength spacingbetween or among the plurality of fiber laser sources results in anapplication having higher conversion efficiency and lower noise figure,which are both highly desirable in many applications including opticalsignal communication systems.

The grating periods of fiber gratings 16, i.e., G₁, G₂ . . . G_(n), areselectively transmissive of different wavelengths λ₁, λ₂ . . . λ_(n)permissible within the gain spectrum of fiber lasers 14. For example,laser L₁ may operate at a wavelength of 1080 nm, L₂ at 1085 nm, L₃ at1190 nm, etc., and L_(n) at 1100 nm. The output from multiple fiberlasers 14 are combined through a wavelength division multiplexer (WDM)18, such as a dichoric mirror or a n×n fused fiber coupler. If a fusedcoupler, for example, is employed as the WDM device, the output of thedouble clad fibers of lasers 14 need be fused to single mode fiberswhich are used in forming the coupler.

System may be combined with additional such systems with their outputsmultiplexed together to form a single high power light output. Themultiple wavelength outputs are combined, producing a high powermulti-wavelength output on WDM coupled output monomode fiber 19. Suchmultiple wavelength outputs are particularly useful for applicationssuch as for pumping a double clad fiber amplifier and for applications,such as material processing, that are insensitive to specific wavelengthbandwidth operation. An example of the former application comprisesfiber laser L₁ providing a wavelength λ₁=980 nm and L₂ providing awavelength λ₂=1480 nm with their combined outputs 19 utilized forpumping an Er doped double clad fiber amplifier employed for amplifyinga 1.55 μm signal. For an Er doped fiber amplifier, wavelengths presentedby lasers L₁, L₂ . . . L_(n) within the range of 960 nm to 1000 nm (±20nm of 980 nm) can be combined at WDM 18 for stimulated emission of theEr doped fiber amplifier. A technique for obtaining a 980 nm output froma Yb doped double clad fiber laser is illustrated in the patent toStephen G. Grubb U.S. Pat. No. 5,530,710, which patent is incorporatedherein by reference thereto. The Yb doped, double clad fiber laser mayemit radiation in the wavelength range of 1060 nm to 1100 nm. However,with the employment of a thin second rare earth Sm doped claddingsurrounding the Yb doped pumped core, emission of any wavelength withinthe 1060 nm to 1100 nm band can be eliminated or suppressed in favor ofenhanced emission at 980 nm. This 980 nm emission, along with the 1480nm emission, are provided, via WDM 18, to the pump core of the Er dopeddouble clad fiber amplifier shown at 16 in FIG. 1 of patent U.S. Pat.No. 5,530,710.

Examples of the just mentioned applications are thermal appliedapplications such as thermal printers, material processing or markingsuch as laser cutting, and trimming and identification, and medicalsurgical applications such as tissue incision and cutting. The fiberlaser system of FIG. 1 can provide power delivery in excess of 1 wattper source which are necessary for modern thermal printing systems. Suchexamples will be explained later in reference to FIG. 24.

By combining the power, for example, of tens of fiber lasers capable ofpower outputs of 10 to 20 watts of individual output power, severalhundreds of watts (or kilowatts in the case of combined peak powers frompulsed applications of such systems to be explained later) of opticalpower can be delivered to monomode output fiber 19. However, there areconcerns that present power handling capabilities of existing singlemode fibers will not be able to handle such high power levels. Adestructive effect presently described or identified as a “fiber fuse”has been observed to occur in transmission of high power levels of theorder of several watts in single mode fibers. The effect ischaracterized by a bright visible light that propagates in a single modefiber beginning at a point of its initiation back toward the lasersource. Examination of the fiber after this effect has shown that thefiber core is partially or completely melted. The initiation of thefiber fuse effect is prevalent at such high powers in standard glassfibers with Ge, P or Al index raising modifiers. This effect is ofparticular concern relative to this invention since WDM combining via asingle mode fiber WDM provides powers of tens of watts to severalhundreds of watts to a single mode fiber core. Pure undoped silicafiber, however, is an ideal transmission media for fiber 19, i.e.,single mode, pure SiO₂ core fibers, because these fibers exhibit highestlaser damage threshold at high power laser radiation levels. Also, someWDM couplers 18 are typically made from germanosilicate fibers andlikely also have an upper power limitation due to the fiber fuse effect.In the utilization of this invention, a single mode fiber 19 or WDMcoupler 18 utilizing pure silica, undoped cores, in the case of a fusedcoupler, are preferred. Also, the cladding surrounding the single modecore may include a fluoride. The index profile in FIG. 2 is one suchexample of a preferred fiber for handling high power levels. Therefractive index profile of the fiber includes index depressed, silicacore fibers because the power handling capability is greatly improved.The fiber comprises a SiO₂ core 15 and an outer SiO₂ cladding 21 ofsubstantially the same, relatively high refractive index. Betweencladding 21 and core 15 is a depressed index cladding 17, such as, forexample, a fluorine depressed cladding.

The wavelength separation between fiber lasers 14 capable of being WDMcombined may be as low as about 1 nm to as high as about 20 nm or more.Narrow wavelength separation achieved through the fiber gratings isdesired because more fiber sources can be multiplexed for a given dopedfiber absorption band. However, when the wavelenglth separation is verysmall, there can be losses due to too much spectral width overlapbetween adjacent fiber laser sources. In the case of Yb doped cores forfiber lasers 14, the absorption bandwidth is available so that a powerscaling system comprising twelve fiber lasers 14 with fiber gratingsproviding pass wavelengths in 5 nm increments is readily achievable.Also, the more combining of multiple fiber laser sources with differentoperating wavelengths to achieve greater power levels in a single outputfiber, the greater the losses that can be encountered, such as SRSscattering. Therefore, in order to reduce such losses, it may bepreferred to have sufficient spatial wavelength separation among theseveral fiber laser outputs so that the SRS loss mechanism substantiallydisappears or is substantially reduced. This sufficient spatialwavelength separation may be 20 nm or more.

Power scaling system 10 may be used even in applications that require anarrow spectrum of operation, such as in the case of frequency doublingapplications through proper selection of spatial wavelength separationbetween fiber lasers 14 and with phase matching conditions provided inthe nonlinear conversion device.

A further alternative embodiment to FIG. 1 is the employment of aplurality of fiber lasers having different fiber lengths, without theuse of fiber Bragg gratings to obtain specific output wavelengths oflight, so that different wavelengths will be achieved among the fiberlasers due to different levels of saturable absorption established ineach laser governed by their fiber length. This approach can be appliedrelative to any fiber that contains a quasi-three level dopant. Aspreviously mentioned, Yb doped fiber lasers are preferred because of itsbroadened homogeneity and the large gain spectrum of Yb material, forexample between 1060 nm to 1120 nm, or its combination with other rareearth materials, such as in the case of Er. Er and Yb are three levelsystems and, therefore, exhibit saturable absorption. For Yb, thesaturable absorption decreases with wavelength. At wavelength 1060 nm,the saturable absorption is very strong and decreases to near zero at1120 nm. For a relatively long fiber capable of absorbing all the pumppower, the operational wavelength of the Yb laser fiber will berelatively long, such as 1100 nm or more, due to the saturableabsorption of shorter wavelength light at or near the output end of thefiber where the fiber is not strongly pumped (of course, provided thatthere is no pump source at that fiber end). However, if the fiber isshortened, the pump power at the output end of the fiber will naturallybe higher. Consequently, the saturable absorption at shorter wavelengthswill be lower and thereby forcing the fiber laser to operate at shorterwavelengths within the Yb gain bandwidth where the gain of the fiber ishigher. Thus, in the embodiment shown in FIG. 1, fiber lasers 14 areprovided without gratings 16, G₁, G₂ . . . G_(n), and the fiber lengthsof the respective fiber lasers L₁, L₂ . . . L_(n) vary from one anotherin length to provide a plurality of fiber lasers 14 each operating at adifferent wavelength within the gain spectra of the rare earth materialsemployed. The outputs from these fiber lasers are combined via WDM 18.As a specific example, for 130 μm diameter Yb doped laser fiber, thewavelength for a 20 meter length is 1092 nm; for a 15 meter length is1090 nm; for a 12.5 meter length is 1087 nm; and for a 10 meter lengthis 1086 nm.

It can be seen that the total available bandwidth for WDM combining canbe increased by providing a plurality of fibers with different lengthsthereby increasing the total available bandwidth of fiber lasers 14.This arrangement can be further optimized by increasing the pump powerfor each of the fiber lasers 14, such as, for example, by providing apump reflector mirror or grating respectively at or within the fibernear its output end to reflect and return unused pump light, or bypumping the fiber laser from both ends, or by pumping the fiber withrespective WDM combined pump sources at periodic locations along thefiber length.

An optimized configuration from the standpoint of providing the widestbandwidth WDM combining scheme would be comprised of a fiber arraycomprising fibers with different fiber lengths, with different pumppower launching conditions, such as pump power launching at the fiberends or along the length with more pump sources provided as the fiberlengths become shorter in length, and with pump power feedback providedthrough incorporation of pump feedback gratings at both ends of eachfiber laser.

Alternatively, instead of fiber lasers 14, fiber amplifiers may beutilized with a plurality of pumping sources, i.e., gratings 16 are notutilized and, instead, pump sources P₁, P₂ . . . P_(n) are provided tobe different semiconductor high power light sources operatingrespectively at different wavelengths which wavelengths are all withinthe absorption band of the fiber amplifier. In FIG. 3, in scaled powersystem 10′, pump sources P₁, P₂ and P₃ have respective output gratingsG₁, G₂ and G₃ that provide for respective wavelength outputs λ₁, λ₂ andλ₃ that are combined via WDM device 18A. The output of device 18A isprovided to amplifier 14A which has an active dopant core for amplifyingthe wavelength-combined pump source light providing an output on fiber19 of high power. If amplifier 14A is a double clad fiber, then fiber 19is provided as a single mode fiber which is fused to the output end ofthe double clad fiber.

The amplified, different wavelength outputs of amplifiers 14 arecombined into a single high power output via WDM 18. Although only twoamplifiers 14A and 14B are shown in FIG. 4, it will be evident to thoseskilled in the art that additional amplifiers may readily be employedand combined via a WDM device 18C. Fiber scaling system 10″ comprisestwo system legs 10A and 10B each respectively having a plurality oflaser pump sources P₁, P₂ and P₃ and sources P₄, P₅ and P₆, withrespective output gratings G₁, G₂ and G₃ and G₄, G₅ and G₆ providing sixdistinctive output wavelengths of radiation which are combined byrespective WDM devices 18A and 18B. The outputs of WDM devices 18A and18B are respectively coupled to fiber amplifiers 14A and 14B havingactive dopant cores for amplifying the wavelength-combined pump sourceradiation. The output of amplifiers 14A and 14B are then combined by WDM18C as a single high power output on fiber 19, such as single mode,undoped silica fiber.

Another embodiment of this invention is shown in generic form in FIG. 5.FIGS. 6-8 are detailed examples of this form as applied to a fiberamplifier and the use of biconical fused couplers that provide forcoupling of spatially close wavelengths. Fiber laser system 40 is apower scaling design utilizing a plurality of fiber laser sources 37,pumped by other pump sources, such as WDM combined laser diode sourceson fiber 36, to provide a plurality of different wavelength outputsλ₁-λ₈ which are WDM combined in pairs forming a tree-like pattern at 39Aand 39B and extending to a final combined pair of outputs, λ₁,-λ₄ andλ₅-λ₈, at WDM device 41. Output transmission fiber gratings 38, aspreviously indicated, are fabricated employing the ultraviolet phasemasking or plural beam interference technique and can be manufacturedwith better than 0.1 nm wavelength accuracy and are very stable with lowinsertion loss which makes them ideal for the application of thisinvention. The outputs of the plural fiber lasers 37 are combinedemploying a combination of WDM devices 39A which may be comprised ofcommercially available fused biconical couplers, such as from Canstar orIP Fiber Devices. The advantage of such a system is that the cascadedtree pattern permits a large amount of fiber laser wavelengths to becombined, as provided over the extended gain spectra of the doped fiber.Also, note that the requirement for WDM couplers decreases by a factorof two when moving down the tree pattern toward sources 37. As explainedpreviously, the pump sources, which are not stabilized in wavelength, donot permit the utility of WDM combining a plurality of pump sourcesnarrowly spaced in wavelength. The use of wavelength-stabilized pumpsources allow the use of pump lasers with a very narrow wavelengthspacing, such as low as 5 nm or less for fiber laser sources 37.

FIGS. 6 and 7 are detailed examples of scaling of pump sources, similarto FIG. 5, but are related to a particular application, which here isamplification of a communication signal. Fiber amplifiers withwavelength multiplexed pump sources have been disclosed in the past, asmention in the Background, relative to U.S. Pat. Nos. 5,185,826 and5,287,216. However, in the disclosure of the embodiments in thesereferences, the pump sources are not stabilized in wavelength and, as aconsequence, it is not possible to provide a plurality of pump sourcesthat are narrowly spaced in wavelength that can be readily combined toprovide added optical power for signal amplification. The use ofwavelength-stabilized pump sources as disclosed in the embodiments ofFIGS. 1-7 allow the use of pump lasers with a very narrow wavelengthspacing, such as low as 5 nm or less, such as for 980 nm semiconductorpump sources, whereas in the case of the above mentioned references, thenarrowest spacing as exemplified in U.S. Pat. No. 5,287,216 is 29 nm.Without wavelength stabilization, trying to achieve a narrow wavelengthspacing would be impractical, as unavoidable drift of the pumpwavelengths would cause undesired fluctuations of the pump powerreaching the amplifier. Thus, achieving a narrow wavelength spacing isof crucial importance as the absorption cross-section of the employedactive dopant, such as Er, drop off rapidly on either said of their peakabsorption wavelength. Employing a narrower pump wavelength spacingbetween or among the plurality of pump sources results in anapplication, such as in the case of a fiber amplifier, with higherconversion efficiency and lower noise figure, both of which are highlydesirable in optical signal communication systems.

FIG. 6 is directed to scaled pump sources for signal amplification.Fiber laser system 100 comprises a high power Er doped fiber amplifier108 pumped by four wavelength-stabilized Laser diodes 102. Laser diodes102 are 980 nm diodes which have a small band of possible wavelengthoutputs between about 970 nm to about 990 nm which are within theabsorption band of amplifier 108 to amplify a signal at about 1550 nmlaunched into the amplifier at input 106. Each individual laser diode102 is stabilized by a fiber grating, G₁-G₄, respectively, whichprovides feedback to the diode at a prescribed wavelength. In the casehere, the peak wavelengths and, therefore, the outputs are,respectively, at λ₁=972 nm, λ₂=982 nm, λ₃=977 nm and λ₄=987 nm. Outputtransmission fiber gratings 104, as previously indicated, are fabricatedemploying the ultraviolet grating writing technique and can bemanufactured with better than 0.1 nm wavelength accuracy and are verystable with low insertion loss which makes them ideal for thisparticular application. The outputs cf the four laser diodes 102 arethen combined employing a combination of WDM devices C₁, C₂ and C₃. WDMdevices 105 may be comprised of commercially available fused biconicalcouplers. In FIG. 4, couplers C₁ and C₂ are designed to combine 972 nmwavelength with 982 nm wavelength, and 977 nm wavelength with 987 nmwavelength, respectively. Because the wavelength response ofmultiplexing couplers 105 is quasi-periodic, it is possible for thethird coupler C₃ with a 5 nm wavelength spacing to multiplex the outputsof the first two couplers C₁ and C₂ into a single optical fiber. With aninsertion loss of less than 0.5 dB per fiber coupler 105, it is possibleto obtain close to 300 mW of total pump power from four standard 90 mWlaser diodes 102. This combined output is then combined, via a standard980 nm/1550 nm coupler C₄, to the 1550 nm signal input at 106 andtogether launched into Er doped fiber amplifier 108. With isconfiguration, a 1550 nm output signal of more than 20 dBm can beachieved.

The example of FIG. 7 differs from the example of FIG. 6 in thatwavelength-multiplexed pump lasers 112 are launched at both ends of therare earth doped fiber amplifier 118, i.e., they are counter propagatingpumps. In the case here, a pump wavelength separation of only 2.5 nm canbe achieved employing 5 nm multiplexing couplers 115 comprising couplersC₁ and C₄, similar to coupler C₃ in the embodiment of FIG. 6. Theoutputs of two 980 nm diode lasers 112 at a first or input end ofamplifier 118, having respective output fiber gratings at peakwavelengths λ₁=976 nm and λ₂=981 nm, are combined via fused biconicalcoupler C₁. The output of coupler C₁ is coupled, via narrow bandwavelength blocking filters 117, F₁ and F₂, to fiber amplifier 118 viaoptical coupler C₂ together with 1550 nm signal input at 116, and theseinputs are launched into amplifier 118. The outputs of two other 980 nmdiode lasers 112 at a second or output end of amplifier 118, havingrespective output fiber gratings at peak wavelengths λ₁=978.5 nm andλ₂=983.5 nm, are combined via fused biconical coupler C₄. The output ofcoupler C₄ is coupled, via narrow band wavelength blocking filters 117,F₃ and F₄, to amplifier 118 via optical coupler C₃. The key to thisdesign is the presence of filters 117 which isolate thecounter-propagating pump lasers from one another. Without filters 117,counter pumping lasers 112 would interact and tend to destabilize oneanother. As examples, filters 117 may be non-reflecting mode-convertingBragg gratings in dual-mode fiber capable of achieving the narrowbandwidths required for these filters. See the paper FB3 of Strasser etal., Optical Fiber Conference '97, which is incorporated herein by itsreference. In case here, couplers C₁ and C₄ are designed to combine 976nm wavelength with 981 nm wavelength and 978.5 nm wavelength with 983.5nm wavelength, respectively. Couplers C₂ and C₃ are broadband 980nm/1550 nm combining couplers. As to filters 117, filter F₁ is designedto block a 978.5 nm peak wavelength; filter F₂ is designed to block a983.5 nm peak wavelength; filter F₃ is designed to block a 876 nm peakwavelength; and filter F₄ is designed to block a 981 nm peak wavelength.These filters block any counter propagating pump light which mayde-stabilize the selected wavelength operation of the pump lasers viatheir output transmission fiber gratings.

The embodiments of FIGS. 6 and 7 may, of course, be extended todifferent numbers of pump lasers or to other types of rare earth dopedfibers such as fiber amplifiers doped with Yb or Nd. Further, whilethese two embodiments relate to applications for high power fiberamplifiers, the same kind of pumping schemes can also be used fordesigning high power fiber lasers such as in the case illustrated inFIG. 1.

Reference is now made to another detail example of this inventioncomprising a power scaling system 800 which is illustrated in FIG. 8.Scaling system 800 comprises a plurality of double clad fiber lasers L₁,L₂, L₃ and L₄ indicated as laser devices 804, 806, 808 and 810, e.g.,fiber lasers with Yb doped cores having a wavelength band of about 1060nm to 1150 nm. Fiber lasers L₁-L₄ are respectively pumped by pumpsources 802, i.e., P₁-P₄, which may, for example, be a high power laserdiode, a laser diode array or a double clad fiber laser pumps. Each ofthe fiber lasers L₁-L₄ includes an optical cavity including appropriatefeedback such a mirror 803 at one end of the cavity that is transparentto the pump light of pumps 802 in the forward direction but reflectsthat light in cavity in rearward direction. The other end of the lasercavities respectively includes a Bragg grating 814, 816, 818 and 820,each having a different transmission wavelength G₁, G₂, G₃ and G₄ whilereflecting all other wavelengths back into the laser cavity. In theparticular example here, all these transmission wavelengths are withinthe operating bandwidth of the Yb doped fiber lasers L₁-L₄. For example,fiber grating Gi of laser L, is set for transmission of λ₁=1060 nm,fiber grating G₂ of laser L₂ is set for transmission of λ₂=1080 nm,fiber grating G₁ of laser L₃ is set for transmission of λ₃=1070 nm, andfiber grating G₄ of laser L₄ is set for transmission of λ₄=1090 nm. Inpairs of fiber lasers L₁ & L₂; L₃ & L₄, the transmission wavelengths areprovided to have about 20 nm wavelength spacing within the gain spectraof the Yb doped fibers or all together may be separated in wavelength ofabout 10 nm or less. The advantage of this wider wavelength separationof WDM combined pairs of fiber laser outputs of 20 nm or more is that iteffectively reduces noise, particularly SRS scattering, which increaseswith increasing combine output power. This noise can be effectivelyeliminated between pairs but the availability of combining wavelengthoutputs with less than 20 nm wavelength separation, e.g., 10 nm or less,is still effectively accomplished throughout the power scaling fiberlaser system.

The pairs of fiber lasers L₁ & L₂; L₃ & L₄ have their outputs combinedthrough the employment of WDIM couplers 822, 824, respectively. Such WDMcouplers for this embodiment as well as other embodiments herein thatutilize fused biconical couplers are commercially available, forexample, from Canstar and IP Fiber Devices as well as others, capable ofbeing set to couple and transmit different wavelength inputs of highpower, such as 10 W or greater. These fused biconical couplers are made,relative to one example, by twisting two fibers together and fusing themat high temperature while drawing the fibers so that the cores narrow atthe formed drawn-out region permitting mode expansion and mixing intothe fiber core. In the case here of multimode fibers, the high ordermodes will easily interact with the core of the twisted adjacent fibertransferring optical power. The amount of transfer depends upon thedegree of coupling, taper length at the point of coupling, and splittingratio. Drawing of the twisted, heated fibers is terminated when thedesired periodic variation of the splitting ratio with taper length isachieved. Since splitting ratio is wavelength dependent, differentwavelengths of light presented at the two input ports of the coupler canbe designed to be WDM combined. As an example, in FIG. 8, coupler 822has a variation in the splitting ratio for wavelength combining of λ₁and λ₂ as inverse transmission compliments at its respective ports 1 and2 since the transmission in each arm of the coupler is periodic inwavelength according to the designed splitting ratio so that the opticalpower of the outputs from fiber lasers L₁ and L₂ are combined as λ₁ andλ₂ on port 1 of fused coupler 822, as shown in the spectral responsediagram accompanying coupler 822 in FIG. 8. The same is true in the caseof fused coupler 824 where its variation in the splitting ratio forwavelength combining of λ₃ and λ₄ as inverse transmission compliments atits respective ports 1 and 2, as shown in the spectral response diagramaccompanying coupler 824 in FIG. 8. In turn, the outputs of couplers822, 824 form limbs or inputs to fused biconical coupler 826 which hassplitting ratio variations design for wavelengths at its port 1 andwavelength inverse transmission function λ₃ and λ₄ at port 2 so that thecombined wavelengths and transferred power at its port 1, i.e., output41 is λ₁, λ₂, λ₃ and λ₄, as shown in the spectral response diagramaccompanying coupler 826 in FIG. 8.

It will be seen that upon examination of FIG. 8, fused biconicalcouplers of the type illustrated at 822, 824 and 826 can be cascaded ina tree-limb fashion back toward fiber laser sources L₁-L₄ so that anynumber of such sources operating at different wavelengths within theoperating wavelength band of the rare earth dopant utilized can be WDMcoupled employing the combining technique provided by these fusedbiconical couplers. The result is that the outputs of a plurality offiber lasers, each having a different wavelength, are WDM combined toprovide a single output of high power and brightness. To be noted isthat the requirement for WDM fused couplers decreases by a factor of twoas one progresses rearward toward laser sources L₁-L₄.

In previous embodiments, mention has been made of the use of fusedbiconical couplers such as in the Cases of FIGS. 6, 7 and 8. Suchcouplers can also be designed so as to have a transmission band topermit passage of a narrow band of light wavelengths falling with theband. In FIG. 9, fused biconical coupler 130 comprises three undopedsilica core fibers B₁ and B₂ which have been either aligned together ortwisted together, then pulled and heated to fuse them together. In FIG.9, they are shown aligned and then fused and pulled together. With aproper variation of the splitting ratio, which is dependent upon thedegree of coupling and taper length, a port-to-port transfer can beprovided where the transmission peaks, λ₁ and λ₂, are extended such asabout ±10%, as illustrated at 132 in FIG. 9 for respective wavelengthbands of λ₁ and λ₂. Thus, coupler 130 provides for a transmissionfunction of a bandwidth of wavelengths λ₁ and λ₂ about the centerwavelength with reasonable, high level coupling efficiency of about ±10%of the designated center wavelength.

Reference is now made to FIG. 10 to highlight further advantages inemploying fiber laser pump sources which are WDM combined dealing withtheir replacement. Fiber laser system 120 comprises a plurality ofsemiconductor or fiber laser sources 122 each having a coupledtransmission output fiber grating 124 respectively providing outputwavelengths of λ₁, λ₂ and λ₃, which are WDM combined via WDM device 127,producing a single output on fiber 128, which output is applied toapplication 129. Sources 122 can also be a plurality of semiconductorlaser diodes or arrays. In the case here, however, quickconnect/disconnect, optical connectors 126 are provided in the opticalfiber lines between gratings 124 and WDM device 127. If catastrophic orother damage befalls any one of fiber laser sources 122, the damagedsource can be removed in the field and replaced with a source operatingwith the same wavelength grating 124 or at least operating with awavelength output differing from any other of the sources 122. In orderto remove any necessity of matching preexisting wavelength sources orprovide a laser source 122 that has an appropriately differentwavelength output for WDM combining along with the other remainingsources 122, the source grating can be permanently positioned betweenthe couplers 126 and WDM device 127 at 125 so that the appropriategrating does not become a required decision making factor as to whatwavelength transmission is acceptable for WDM device 127 as long as thesubstituted fiber laser source 122 has a gain spectrum matching thein-line grating wavelength of fiber grating 127. This is a particularlygood quick-connect application utilizing permanently in-place fibergratings because of their proven high environmental stability over time.It should be noted that in FIG. 10, either grating 124 exists or grating125 exists at the output of any particular source 122, i.e., thesegratings obviously do not exist in pairs.

Also, it should be noted that a quick connect/disconnect coupler 126 mayalso be placed in output fiber 128 so that the whole power scaling unitcomprising WDM combined sources 122 can be replaced due, for example, tofailure.

Lastly, if application 129 is wavelength sensitive to a particularwavelength bandwidth, a broadband fiber grating may be formed in outputfiber 128 to select the transmitted bandwidth of desired wavelengths.

A specific application 129 in FIG. 10 may be, for example, power scalingof an inner cladding pumped double clad fiber MOPA structure of the typeshown, for example, in U.S. Pat. No. 5,473,622 to Stephen G. Grubb,which is incorporated herein by its reference. Moreover, the differentwavelength outputs of a plurality of such fiber MOPA devices, such asshown in either FIGS. 2, 3 or 4 of that patent, whether individuallypumped by one or pumping sources combined in the manner as explainedherein, can be WDM combined to provide a single high power output on asingle mode fiber employing WDM devices to combine their respectiveoutputs together.

Reference is now made to FIG. 11 which discloses another embodiment ofthis invention comprising fiber laser system 140. System is basicallythe same as system 10 illustrated in FIG. 1 in that a plurality of fiberlasers 142 have in-line fiber gratings 144 for providing stabilizedoutputs of peak wavelengths of λ₁, λ₂ and λ₃ for WDM combining at WDMdevice 146 forming a single output on fiber 147 for coupling to anapplication 148. The difference, however, is that in-line fiber gratings144 are chirped gratings having a periodic bandwidth sufficiently narrowto permit sufficient WDM combining of the outputs of the fiber lasersbut a sufficiently wide bandwidth in order that the individual beamswill be of low noise in order to meet the requirements of an intendedapplication 148. These chirped gratings may be about 1 cm long, forexample and have a wavelength sensitivity of up to several nanometers.Chirped gratings 144 selectively provide for transmission of a narrowbandwidth of wavelengths for WDM combining in a manner similar to thewavelength bandwidths 132 in FIG. 9 except they have a bandwidth of asmaller scale.

In the previous discussion of FIG. 9 above, a fused biconical coupler130 was illustrated for WDM combining of a plurality of radiationwavelengths. Rather than using a fused coupler, another WDM combiningapproach is the employment of a grating mirror as shown in FIG. 12. WDMcombiner 150 comprises a grating mirror 156 which has a gratingreflector surface 158. Light beams from a plurality of laser sources152-155, which may have the same gain spectra, are positioned atdifferent angles relative to the planar extent of surface 158. Outputbeams 152A-155A from laser sources 152-155, albeit semiconductor laseror fiber lasers, are angularly directed onto grating surface 158 at agiven point where they are reflected together at the same angle fromsurface 158 forming a single output beam 157 which may be coupled into asingle mode fiber (not shown). Thus, the angular positioning of sources152-155 relative to planar surface 158 is wavelength determinative oftheir particular output beams. Partial mirror 159 provides for a portionof the light of the respective wavelengths to be reflected back to lasersources 152-155. Thus, because of the different angles and path lengthstaken by the respective beams from these sources, they will all lase atdifferent wavelengths, λ₁, λ₂, λ₃ and λ₄. The angular degree between theangularly positioned sources 152-155 is small, such as a few degrees, toachieve a 5 nm wavelength spacing among the different wavelengths λ₁,λ₂, λ₃ and λ₄. This arrangement is similar to that shown in U.S. Pat.No. 5,379,310 to Papen et al. but differs in not combining the outputbeams for power scaling into a single output beam as well as the mannerof establishing wavelength feedback in the zero order which is also theoutput beam order.

Alternatively, in the case of fiber lasers comprising sources 152-155,the fiber lasers are stabilized in wavelength, λ₁, λ₂, λ₃ and λ₄, withindividual fiber gratings written in each of the fiber lasers. Theiroutputs are then WDM combined with a single mirror grating 158 asillustrated in FIG. 12. In this alternative, feedback mirror 159 is notrequired.

Reference is now made to FIGS. 13 and 14 illustrating a further of thisinvention. Fiber laser system 160 comprises two groups or sets of fiberlasers 163, L_(A1) . . . L_(An), and fiber lasers 165, L_(B1) . . .L_(Bn) with fiber gratings 161 and 167 with the fiber laser wavelengthoutputs established via fiber gratings 166 and 168, which wavelengthsare within the gain spectrum of lasers 163 and 165, respectively. Thedifferent wavelength outputs of fiber lasers 163 and 165 are WDMcombined via respective WDM devices 169 and 170. Fiber lasers of set Aare pumped by laser sources 162 whereas fiber lasers of set B are pumpedlaser sources 164. Fiber lasers of set A comprise a first rare earthdopant or dopants that has a first gain spectrum, A₁-A_(n), asillustrated in FIG. 14 whereas fiber lasers of set B comprise a secondrare earth dopant or dopants that has a second gain spectrum, B₁-B_(n),as illustrated in FIG. 14 wherein the gain spectra of sets A and B areoverlapped or are adjacent to one another forming together an extendedsingle gain spectrum 173. Pump sources 162 have wavelength outputswithin the absorption band of fiber lasers 163 and pump sources 164 havewavelength outputs within the absorption band of fiber lasers 165. Thecombined outputs from WDM devices 169 and 170 are combined by WDM device171 providing a single high power output at 172 in the tens to hundredsof watts.

Reference is now made to another embodiment of this inventionillustrated in FIGS. 15-17. This embodiment comprises power scalingsystem 42 and is similar in concept to power scaling system 800 of FIG.8 except that different wavelength pump laser sources for pumping doubleclad fiber lasers are combined via WDM. The double clad fiber lasers maybe comprised of a double core configuration comprising two concentricsingle mode cores surrounded by a pump cladding core to provide higherpower output from a single double clad, dual core fiber laser comparedto the employment of a single double clad fiber laser.

In FIG. 15, a particular example of an embodiment is shown althoughother different configurations can be readily envisioned by thoseskilled in the art. Power scaling system 42 ,may comprise a Nd:Yb doped,double clad, dual core fiber laser 45 having an optical cavity asdefined gratings 46A, 46B with an emission wavelength of λ_(A)=1060 nm,for example, and a Yb doped double clad fiber 47 having optical cavityas defined gratings 48A, 48B having an emission wavelength of λ_(B)=1080nm, for example.

As shown in FIG. 16, fiber laser 45 has dual single mode corescomprising Nd doped, inner core 51 and outer Yb doped, concentric core52, each of the same refractive index or substantially similarrefractive index, which is surrounded by a substantially rectangularshaped, inner pump cladding 53 having a refractive index lower than thatof cores 51, 52 and having a NA compatible with the NA of the pump lasersources 43A, 43B. An outer cladding 54 and fiber sheath (not shown) areformed on pump cladding 53. The total diameter of both inner and outercores 51, 52 may be D₁ equal to about 7 μm to 8 μm forming a single modedual core. As a specific example, the inner Nd core 51 may about 4 to 5μm and the outer Yb core 52 may be about 2 to 4 μm. The inner core 51may be larger than the outer core 52 and visa versa but together theyare single mode. Also, Nd can be in outer core 52 and Yb in inner core51. Other rare earths can be employed in place of either of these activeelements or a core may be co-doped with active elements such as Er:Yb.Pump cladding 53 may be, as an example, W equal about 250 μm to 370 μmand H equal to about 100 μm to 200 μm.

As shown in FIG. 17, fiber laser 45 has an Yb doped, inner core 55having a high refractive index, surrounded by a substantiallyrectangular shaped pump core 56 having a refractive index lower than therefractive index of core 56 and a NA compatible with the NA of the pumplaser sources 43C, 43D. An outer cladding 57 and fiber sheath (notshown) are formed on pump core 56. Fiber laser 45 is pumped with atleast two semiconductor lasers or laser arrays 43A, 43B havingrespective output wavelengths of around λ₁=810 nm and λ₂=915 nm or 980nm, the former being within the absorption band of Nd and the latterbeing with the absorption band of Yb. A. multiplicity of pump sourcesmay be WDM combined at each pump band. By using a double core withrespective rare earth dopants, two pump bands are available increasingthe total amount of pump power that can be WDM combined into a singlemode fiber. Outputs of pump sources 43A, 43B are combined via WDM device44A (e.g., a dichoric mirror or a directional coupler) and are provideas pump input to fiber laser 45 via pump cladding 53. In the case offiber laser 47, at least two semiconductor lasers or laser arrays 43C,43D are WDM combined with WDM device 44B and are provided as pump inputto fiber laser 47 via pump core 56. Laser sources 43C, 43D haverespective output wavelengths of λ₃=915 nm and λ₄=975 nm, which are bothwithin the absorption band of Yb.

Two differently doped fiber lasers 45 and 47 Nd:Yb dual core doped andYb single core doped) are illustrated here to show how the gainbandwidth can be widened and, correspondingly, increase the number ofpossible different wavelength fiber lasers that can be VJDM combined viaWDM device 49 comprising, for example, a fused biconical couplerproviding the combined power outputs of both fiber lasers 45 and 47 atoutput 50.

Reference is now made to FIG. 18 which shows a further embodiment ofthis invention for power scaling output power employing a plurality offiber amplifiers A₁, A₂ . . . A_(n), only two of which are shown herefor purposes of illustration. Power scaling system 20 comprises aplurality of amplifiers 26, A₁, A₂, etc., which are double clad fibercores doped or codoped with rare earth dopants. The different wavelengthoutputs, λ₁, λ₂ . . . λ_(n), of signal sources 24 are coupled into thecores of fiber amplifiers 26 via beam combiner 25 while the pumpradiation from pump sources 22 is launched into the pump core of doubleclad fiber amplifiers 26 via beam combiner 25. Signal sources 24, S₁ andS₂, are modulated, as illustrated in FIG. 19A, to provide opposingmirrored pulse output and which are then amplified by amplifiers 26.Modulated wavelength, λ₁, λ₂ . . . λ_(n), outputs are combined via timedivision multiplexer (TDM) 28 in a manner that the respective modulatedoutputs of amplifiers A₁, A₂ . . . A_(n) are time-width combined toproduce a cw output as shown in FIG. 19B. TDD 28 may be comprised of,for example, an acousto-optical switch, or an oscillating mirror orprism.

An important feature of this embodiment in FIG. 18 is that by means ofhigh pulse rate modulation of signal sources 24, large peak powers canbe achieved, which when amplified by fiber amplifiers 26 and timemultiplex to provide a cw output 29, provide a highly effective schemefor producing a very high output power, particularly for applicationsnot sensitive to very small fluctuations in the combined, total cwoutput. The peak power in each fiber amplifier 26 is twice the averagepower available from fiber amplifiers 26 because of the energy storagecapacity of the fiber amplifiers. Pulse lengths for the signal sourceoutputs shown in FIG. 19A may range from several microseconds to severalnanoseconds. Lower modulation rates are advantageous for providingsimple low speed switching via TDM 28. A pump system such as source 24and amplifier 26 that may be used with this invention are disclosed inFIG. 1 or FIG. 14 of U.S. Patent (U.S. Ser. No. 08/588,747, filed Jan.19, 1996) and in U. S. Pat. No. 5,657,153, both of which are commonlyowned by the assignee herein and are incorporated herein by theirreference.

FIG. 20 graphically illustrates a further embodiment employing powerscaling system 20 of FIG. 18 for producing a continuous train of highpower pulses rather than a cw output described in connection with FIG.18. In this configuration, the respective modulated outputs 30 and 32 ofsources S₁ and S₂ are received at TDM 28 which time multiplexes theasynchronous amplified outputs to produce the pulse train 34 shown foroutput 29 in FIG. 20. In this alternate embodiment, the switching rateof TDM 28 may be relatively slow while producing a total high frequencypulse stream at output 29.

The particular output operation provided in the embodiments of FIGS.18-20 are particularly advantageous for thermal printer operation whichare more efficient at high peak power levels of operation which iscapable of being provided by these power scaling systems. This isbecause these kind of thermal print operations generally have highthermal diffusion constants requiring short pulse of high incident peakpowers to meet the desired diffusion constants. However, the scaling ofpower of embodiments are also applicable to other applications as wellas printing and marking applications as depicted in FIG. 24. The fiberlaser system of any of these foregoing embodiments may be provided as asingle output on a flexible fiber, such as a single mode, undoped silicafiber, for coupling the high power output from the fiber laser system toa movable carriage 202 of a printer, marker or thermal processing beamdelivery device 200 as depicted in FIG. 24. The carriage may support aprint or marking head for receiving the high power multiwatt outputlight beam for directing the beam to application 204 as the carriage 202is moved relative to application 204, such as moved transversely withrespect to the application. For this kind of application, a single modefiber 206 is sufficiently flexible to moved relative to device 200 toprovide for flexible beam delivery from fiber laser system 208. Such asingle mode flexible fiber removes speckle and noise feedback providinga stable radiation intensity delivery system. Examples of applicationsare light sensitive mediums in a thermal or xerographic printer systems;or in material processing, such as, cutting, welding and holefabrication in metals and alloys; or thermal heat treatment via ascanned beam across a surface to be heat treated to provide functions ofsurface layer removal or surface annealing; or in surface marking suchas in surface tattooing or writing. The outputs, for example, of atleast two double clad fiber lasers with power outputs of about 1 watt ormore may be VTDM combined into a single fiber output. The use of asingle mode fiber with such high power levels in the tens of watts hasthe advantage of providing single mode high beam quality allowing for alarge depth of focus useful in these types of applications as will beappreciated by those skilled in the art.

Reference is now made to FIG. 21 illustrating another embodiment of thisinvention. Fiber laser system 180 for providing power scaling includesthe combination of wavelength combining of radiation via wavelengthdivision multiplexing (WDM) with polarization beam combining ofradiation via polarization beam multiplexing (PBM). PBM is anothermanner of combining radiation into a single output. This principle isknown in the art, as previously indicated relative to the U.S. Pat. No.5,311,525 to Pantell et al. wherein in column 14, there is a discussionof such optical devices for coupling of optical energy or radiationbetween two different sets of polarized radiation modes, such as the xand y polarization modes for fundamental or second order modes of eitherLP₀₁ or LP₁₁. However, in FIG. 21, a combination of WDM and PBM isillustrated in fiber laser system 180 comprising two legs, a first legcomprising a plurality of fiber lasers 182 with respective inline outputfiber gratings 183 and a second leg comprising a plurality of fiberlasers 184 with respective in-line output fiber gratings 185. Fiberlasers 182 are coupled to a first WDM device 186 and fiber lasers 184are coupled to a second WDM device 187. A primary difference between thefirst and second leg groups is that the wavelengths of respective leggroups are orthogonally polarized. A x-polarized LP₀₁ mode is prevalentin the first leg of system 180 which is maintained with the use ofpolarization maintaining fibers for fiber lasers 182, such as with apolarizing element providing a small loss induced for one polarizationby means of a short piece of polarizing fiber, or polarizationmaintaining fibers for the coupling fiber between lasers 182 and WDMdevice 186 containing fiber gratings 183. On the other hand, ay-polarized LP₀₁ mode is prevalent in the second leg of system 180 whichis maintained with the use of polarization maintaining fibers for lasers184, such as with a polarizing element providing a small loss inducedfor one polarization by means of a short piece of polarizing fiber, orpolarization maintaining fibers for the coupling fiber between lasers184 and WDM device 186 containing fiber gratings 185. The outputs of WDMdevices 186 and 187, therefore, contain a plurality of combinedwavelengths λ_(T1) and λ_(T2) with respective x and y polarized LP₀₁modes in polarization maintaining (PM) fibers 188 and 189. PM fibers 188and 189 are coupled to polarization beam combiner 190 which PBM combinesthe polarized mode leg sets; of wavelength outputs as a single output onfiber 191.

In the description of the previous embodiment, reference has been madeto fundamental order polarization modes. However, fundamental ordermodes or second order modes or first order modes per se can berespectively PBM combined such as the combinations of horizontal,vertical circular or radial polarization modes.

Reference is now made to a further embodiment of this inventionillustrated in FIG. 22 comprising power scaling system 60. System 60 issimilar to power scaling system 10 in FIG. 1 except that the bandwidthof combined fiber laser sources is increased through the use of Ramanlasers either in the form of a single Raman fiber laser that provides aRaman shift or a series of coupled Raman shift fiber lasers providing aplurality of Raman shifts. An exemplary Raman laser is shown in FIG. 1of U.S. Pat. No. 5,323,404 to Stephen G. Grubb, and in the article ofStephen G. Grubb et al. entitled, “1.3 μm Cascaded Raman Amplifier inGermanosilicate Fibers”, Paper PD3, Optical Amplifiers and TheirApplications, Technical Digest, Optical Society of America, Washington,D.C. (1993), which references are incorporated herein by reference. Asin the case of system 10, a plurality of fiber lasers, such as Yb dopedfiber lasers 62A, 62B, pumped by semiconductor or fiber pump lightsources 61A, 61B, provide power output of different wavelengthsdetermined by respective fiber gratings 64A and 64B, here shown asseparate by 10 nm, respectively, 1110 nm and 1120 nm, which are withinthe gain bandwidth of Yb, as indicated at 69 in FIG. 23. Thus, for thepurposes of this explanation, Yb fiber lasers 62A, 62B are two suchlasers that may be utilized in the possible combinations from the Ybgain bandwidth 69 (FIG. 23), which has a bandwidth of around 100 nm. Inorder to increase this bandwidth further so as to provide additionalfiber laser outputs for WDM combining, Raman shifting of wavelengthswithin band 69 can be utilized which is exemplified in conjunction withRaman fiber lasers 63A and 63B of FIG. 22. For example, Yb doped fiberlaser 62C, pumped by semiconductor or fiber pump source 61C, has a fibergrating 64C set for a wavelength of about 1100 nm. The output of laser62C is fusion spliced to Raman laser 63A comprising a single mode fibersimilar of the type shown in the above mentioned article of Grubb et al.and having refractive index fiber grating pairs 65A, 65B and 66A, 66Bwith center wavelengths respectively at 1130 nm and 1160. These gratingsare all highly reflective except that grating 66B permits substantialtransmission of wavelength light at 1060 nm. Gratings 65A, 65B at centerwavelength 1130 nm provide for internal reflected and buildup forstimulation of light generation at center wavelength 1160 nm. The 1160nm output at 61D from Raman laser 63A can be, in turn, provided as input61E to another Raman laser 63B to provide a second Raman shift of theinput wavelength light. Raman laser 63B includes refractive index fibergrating pairs 67A, 67B and 68A, 68B with center wavelengths respectivelyat 1190 nm, and 1220 nm. Gratings 67A, 67B at center wavelength 1190 nmprovide for internal reflected and buildup for stimulation of lightgeneration at center wavelength 1160 nm with output from highlytransmissive grating 68B of light at 1220 nm. Effectively, the Ramanshifted outputs of Raman lasers 63A and 63B are provided to a VWDMdevice for WDM combining and ultimately on a single output as combinedwith other different wavelength outputs in FIG. 22. Thus, the wavelengthband can be effectively expanded, as illustrated in FIG. 23, toadditional shifted wavelengths beyond the gain bandwidth 69 for Yb foradditional WDM combining for power scaling applications or otherapplications requiring a plurality of different wavelengths. The Ramanshift of Raman laser 63A is indicated at line 69A in FIG. 23. AdditionalRaman shifts can be attained in the same manner as previously indicatedusing multiple Raman laser stages, for example, by the Raman shift from1110 nm, such as from Yb fiber laser 62A, to 1170 nm, indicated by line69B, which may be further Raman shifted as indicated at line 69D in FIG.23. The second Raman shift provided by Raman laser 63B is indicated atline 69C in FIG. 23.

Reference is now made to FIG. 25 which discloses a further embodiment ofthis invention. In FIG. 25, laser sources 252, which may besemiconductor lasers (e.g., DFB, α-DFB or DBR lasers) or fiber lasers(e.g., double clad fiber lasers), produce light of differentwavelengths, here λ₁ and λ₂, which are WDM combined via wavelengthdivision multiplexor device 253 to produce a common output on fiberoutput line 256. In output line 256 are broad band fiber gratings 254,λ_(B1) and λ_(B2), which are respectively partially reflective of lightfrom laser sources 252, λ₁ and λ₂. Each fiber grating 254 thus receivesthe light from a respective source via WDM device 253 to stabilize itswavelength of operation to a narrow linewidth output at a wavelengthdifferent from the wavelength output of the other laser source or of anyother laser source. The difference in this embodiment from previousembodiments is that WDM device 253 is coupled between laser sources 252and broad band fiber gratings 254 which enhances the stabilization oflaser sources 252. Over time or during operation, WDM device may changein characteristics due to, for example, aging and ambient changes, suchas temperature, so that polarization and/or wavelength preset responseconditions shift from the prescribed polarization or wavelengthconditions of sources 252. If the stabilization gratings for sources 252are positioned between the output of these sources and the WDM device,polarization and/or wavelength response shifts of the WDM device fromits original setting conditions will not be as responsive in combiningall of the light output of laser sources 252. As a result, there will behigher losses experienced in the combined output of light on line 256.To correct for this condition, broad band fiber gratings 254 may beemployed, which are positioned, not at the output of sources 252, butrather at the output of WDM device 253. Thus, if any polarization orwavelength response shifts, due to aging or environmental effects on WDMdevice 253, occur, then respective laser sources will automatically seekoptimal polarization and wavelength operation within the respectivebandwidths of fiber gratings 254 leading to enhanced operatingstabilization of laser sources 252 in spite of WDM device responseshifts over time. As a result, optimum efficiency of light combiningfrom sources 252 is continually maintained.

Fiber gratings 252 may be periodic or chirped, such as shown in FIG. 11,providing for a larger broader bandwidth response. WDM device 253 may bea directional coupler or other type of combiner sensitive to aging andenvironmental changes, such as dichoric mirror couplers or a multiple(n×n) fused fiber coupler. Also, the structure of FIG. 25 may beutilized in other embodiments of this invention, such as, for example,FIGS. 1, 4, 5, 6, 8, 10, 13 and 21, in multiple form for the multiplecoupling of a plurality of different wavelength outputs.

Although the invention has been described in conjunction with one ormore preferred embodiments, it will be apparent to those skilled in theart that other alternatives, variations and modifications will beapparent in light of the foregoing description as being within thespirit and scope of the invention. Thus, the invention described hereinis intended to embrace all such alternatives, variations andmodifications as that are within the spirit and scope of the followingclaims.

What is claimed is:
 1. A fiber laser system for scaling the optical power to a common output, comprising: a plurality of fiber lasers each comprising a fiber having an inner cladding surrounding an active dopant core and an outer cladding surrounding the inner cladding; at least one pumping source coupled to the inner cladding of each of said fiber lasers; a reflector to stabilize the wavelength of operation of at least one of said fiber lasers to a wavelength different from the wavelength output of any other of said fiber lasers; and a wavelength division multiplexer (WDM) device coupled to receive and combine said different wavelength outputs as a single output.
 2. The fiber laser system of claim 1 wherein said reflector comprises a fiber grating.
 3. The fiber laser system of claim 1 wherein said WDM device and its single output are coupled into a single mode optical fiber.
 4. The fiber laser system of claim 1 wherein said wavelength operation of said at least one fiber laser is a narrow linewidth output.
 5. The fiber laser system of claim 1 wherein at least some of said fiber lasers have an active dopant core consisting of Er⁺, Nd⁺, Yb⁺, Nd⁺/Yb⁺, or Er⁺/Yb⁺.
 6. The fiber laser system of claim 1 wherein said WDM device is comprised of single mode fibers and its single output is coupled into a single mode optical fiber.
 7. The fiber laser system of claim 1 wherein at least some of said fiber lasers comprise fibers having an index depressed cladding between their cores and their inner claddings.
 8. The fiber laser system of claim 1 wherein said WDM device is fabricated from undoped silica to prevent fiber failure due to WDM combined high optical powers.
 9. The fiber laser system of claim 1 wherein said WDM device is coupled to a single mode fiber.
 10. The fiber laser system of claim 9 wherein said single mode fiber has a core comprising undoped silica.
 11. The fiber laser system of claim 1 wherein said fiber grating is chirped providing a periodic bandwidth sufficiently narrow to permit wavelength multiplexing of the outputs of said fiber lasers but sufficiently wide to provide for low noise operation.
 12. The fiber laser system of claim 1 wherein said WDM device comprises at least one selected from the group consisting of a dichroic mirror, a directional coupler and a fused conical coupler.
 13. The fiber laser system of claim 1 wherein said at least one pumping source comprises at least one laser diode having a pumping wavelength within the absorption bandwidth of its coupled fiber laser.
 14. The fiber laser system of claim 1 wherein the wavelength separation between or among said fiber lasers is no more than 5 nm due to said fiber gratings enhancing the number of possible fiber lasers that may be WDM combined.
 15. The fiber laser system of claim 1 wherein the wavelength separation between or among said fiber lasers is no less than 20 nm due to said fiber gratings to avoid stimulated Raman scattering in said fiber lasers.
 16. The fiber laser system of claim 1 further comprising coupling means for releasably securing said fiber lasers to from said WDM device.
 17. The fiber laser system of claim 1 wherein the pumping sources are semiconductor lasers or semiconductor laser pumped fiber lasers.
 18. The fiber laser system of claim 1 further comprising a quick-connect optical coupler in the output fiber either between each of said fiber lasers and its corresponding fiber grating or between each of said fiber gratings and its corresponding WDM device to permit replacement of said fiber laser with retention of its wavelength being predetermined via said corresponding fiber grating.
 19. The fiber laser system of claim 1 wherein said active dopant cores comprise a rare earth dopant having an absorption band broader than about 20 nm.
 20. The fiber laser system of claim 1 wherein said fiber lasers have laser active dopants comprising rare earth material providing wavelength spectra within which wavelengths are selected via a set grating period of their respective Bragg gratings, at least some of the outputs of said double clad fiber lasers coupled to a Raman fiber laser to provide Raman wavelength shifting to wavelengths beyond the wavelength spectra provided by said laser active dopants increasing the number of possible different output wavelengths capable of being combined via said WDM device.
 21. The fiber laser system of claim 20 wherein the output wavelength separation between at least some of said fiber lasers is greater than the wavelength spacing required to avoid stimulated Raman scattering in the WDM combination of said different wavelength outputs as a single output.
 22. The fiber laser system of claim 21 wherein said wavelength separation is greater than about 20 nm.
 23. The fiber laser system of claim 1 wherein the output wavelength separation between at least some of said fiber lasers is greater than about 20 nm to avoid stimulated Raman scattering in the WDM combination of said different wavelength outputs as a single output.
 24. The fiber laser system of claim 1 wherein the outputs of said fiber lasers are respectively coupled to a fiber amplifier, the amplified outputs of said fiber amplifiers WDM combined via said WDM device.
 25. The fiber laser system of claim 1 wherein the pumping sources to each of said fiber lasers are separately pulsed to provide high peak powers with their outputs time division multiplexed (TDM) to provide a substantially cw high power output.
 26. The fiber laser system of claim 1 wherein said single output is provided to a printer having a light sensitive medium.
 27. The fiber laser system of claim 26 wherein single output is provided as input into a flexible fiber for delivery to said light sensitive medium separate from the fiber laser system.
 28. The fiber laser system of claim 26 wherein the output end of said flexible fiber is held in a print head which is movable transversely relative to said light sensitive medium.
 29. The fiber laser system of claim 26 wherein said printer is a thermal printer or an ablative printer.
 30. The fiber laser system of claim 29 wherein said system comprises at least two double clad fiber lasers have respective power outputs of approximately 1 watt with their outputs WDM combined.
 31. The fiber laser system of claim 26 wherein said printer is a xerographic printer.
 32. The fiber laser system of claim 1 wherein said single output is provided for materials processing, surface treatment or marking.
 33. The fiber laser system of claim 32 wherein said materials processing comprises cutting or forming holes in metals and their alloys.
 34. The fiber laser system of claim 32 wherein said marking comprises surface tattooing or writing.
 35. The fiber laser system of claim 32 wherein said surface treatment comprises selective surface removal or annealing.
 36. The fiber laser system of claim 32 wherein said single output is coupled into a flexible fiber for delivery to said application.
 37. The fiber laser system of claim 36 wherein said flexible fiber is a single mode fiber.
 38. The fiber laser system of claim 1 wherein at least some of said fiber lasers are doped with active dopants having different gain spectra to extend the range of possible spatial wavelength separation among said fiber lasers via said fiber gratings.
 39. The fiber laser system of claim 1 wherein said reflector comprises a fiber grating of at least some of said fiber lasers, said fiber gratings being chirped to reduce noise but having a grating period still sufficiently narrow to permit wavelength selection and WDM combining via said WDM device.
 40. The fiber laser system of claim 1 wherein said WDM device allows selective wavelengths that can be efficiently combined as said single output.
 41. The fiber laser system of claim 40 wherein said WDM device is a monomode fused taper coupler.
 42. The fiber laser system of claim 41 wherein said WDM device comprises undoped silica core fibers to minimize fiber damage due to high power levels permitting selective wavelength combining of said different wavelength outputs forming said single output.
 43. The fiber laser system of claim 42 further comprising an output fiber of undoped silica for receiving said single output for transmission to an application.
 44. The fiber laser system of claim 40 wherein said fiber lasers include a set of reflectors having a reflection band within ±10% of the peak transmission wavelengths enabled by said WDM device.
 45. The fiber laser system of claim 1 wherein said WDM device is a reflector having a grating on a reflecting surface of said WDM device.
 46. The fiber laser system of claim 1 wherein wavelength spacing of said plurality of outputs of different wavelengths is within 10% of one another.
 47. The fiber laser system of claim 46 wherein said wavelength spacing is within the range of about 2 nm to about 10 nm.
 48. The fiber laser system of claim 1 further comprising a plurality of said systems providing a plurality of said single outputs wherein said beams are of different light polarization modes, a polarization combiner coupled to receive said plurality of said single outputs and combine all of said different polarization outputs into a single output.
 49. The fiber laser system of claim 1 wherein at least some of said fiber lasers have the same output wavelengths with different light polarization modes, and a polarization combiner coupled to receive and combine all of said different light polarization mode outputs into a single output.
 50. The fiber laser system of claim 49 wherein said single output, polarization mode beam and said single output, wavelength beam are WDM combined via said WDM device.
 51. The fiber laser system of claim 1 wherein said fiber lasers have a cross-sectional rectangular pumped core.
 52. The fiber laser system of claim 1 wherein some of said fiber lasers are of different fiber length from others of said fiber lasers.
 53. The fiber laser system of claim 1 wherein at least some of said fiber lasers each comprise a fiber master oscillator with an integrated fiber amplifier, the output of said fiber amplifier provided to said WDM device.
 54. The fiber laser system of claim 1 wherein said at least one pumping source comprises a plurality of laser diodes having different wavelength outputs that are WDM combined and coupled to its respective fiber laser.
 55. The fiber laser system of claim 54 wherein wavelength spacing of said plurality of laser diodes is within 10% of one another.
 56. The fiber laser system of claim 55 wherein said wavelength spacing is within the range of about 2 nm to about 10 nm.
 57. The fiber laser system of claim 54 wherein a fiber grating is coupled between the output of said diode lasers and their WDM combining to provide continuous wavelength stabilization.
 58. The fiber laser system of claim 57 wherein said fiber grating is a chirped grating to reduce noise but has a grating period still sufficiently narrow to permit wavelength selection and WDM combining.
 59. A fiber laser system comprising: a plurality of double clad fiber lasers having a pump core and rare earth doped cores; at least one pumping source coupled to the pump core of each of said fiber lasers; a signal source coupled to the pumped core of each of said fiber lasers, each of said signal sources individually modulated to provide a train of pulses that are out of phase with the train of pulses of other of said signal sources; and a time division multiplexor (TDM) device coupled to receive and combine all of said plural pulse trains as single output source.
 60. The fiber laser system of claim 59 wherein said single output source provides a cw output.
 61. The fiber laser system of claim 59 wherein said single output source provides a pulse output of equally spaced pulses.
 62. The fiber laser system of claim 59 wherein said single output source provides a pulse output of unequally spaced pulses.
 63. A fiber laser system for scaling the optical power to a common output, comprising: a plurality of fiber lasers each having an inner cladding surrounding a rare earth doped core; at least one pumping source coupled to the inner cladding of each of said fiber lasers; a fiber grating formed in an output fiber from at least some of said fiber lasers to stabilize the wavelength of operation of said fiber lasers to a narrow linewidth output at a wavelength different from the wavelength output of any other of said fiber lasers producing a plurality of outputs of different wavelengths; at least one of the outputs of said fiber lasers coupled to a Raman fiber laser to provide Raman wavelength shifting and means for combining the outputs of said fiber lasers to produce a high power common output.
 64. The fiber laser system of claim 63 wherein said combining means comprises a plurality of wavelength division multiplexer (WDM) devices for combining all of said different wavelength outputs as said common output.
 65. A fiber gain source comprising at least two concentric, monomode pumped cores surrounded by an outer pump cladding for pumping of said cores, the first of said pumped cores incorporated a first active dopant and the second of said pump cores incorporated with a second active dopant, and a pumping source for providing pump light to said outer pump cladding and having a wavelength band within the absorption band of both the first and second active dopants.
 66. The fiber gain medium of claim 65 wherein said outer pump core has an elongated cross-sectional configuration.
 67. The fiber gain medium of claim 66 wherein said cross-sectional configuration is rectangular shaped.
 68. The fiber gain medium of claim 65 wherein said first pump core is doped with Nd and said second pumped core is doped with Yb.
 69. A laser system for scaling the optical power to a common output, comprising: a first set of plurality of laser sources having different wavelength outputs; means at the output of each of said laser sources to stabilize their output wavelengths of operation; a plurality of WDM couplers for coupling the outputs of pairs of said laser sources in a tree-like pattern until all of said laser sources are coupled to the common output.
 70. The laser system of claim 69 wherein WDM couplers are coupled between said stabilizing means and said different wavelength outputs.
 71. The laser system of claim 69 wherein said laser sources are semiconductor lasers or fiber lasers.
 72. The laser system of claim 69 wherein coupled pairs of said laser sources have wavelength separations in the range of about 10 nm to 30 nm.
 73. The laser system of claim 69 wherein said WDM couplers comprise fused tapered couplers having splitting ratios and forward transmission functions dependent upon the wavelength emissions of coupled pairs of said laser sources.
 74. The laser system of claim 69 wherein said laser sources comprise at least three optical fiber lasers.
 75. The laser system of claim 69 wherein said WDM coupler comprises a fused conical coupler.
 76. The laser system of claim 69 wherein said stabilizing means is a fiber grating or a fiber chirped grating.
 77. The laser system of claim 69 wherein said WDM couplers are peaked for band transmission within ±10% of the peak laser wavelengths.
 78. The laser system of claim 69 wherein said common output is a first common output is coupled to a first end of a pump core of a double clad fiber amplifier for amplifying a signal input supplied to a single mode pumped core of said fiber amplifier.
 79. The laser system of claim 69 further comprising: a second set of plurality of laser sources; means at an output of each of said second laser sources to stabilize their output wavelengths of operation; a plurality of WDM couplers for coupling the outputs of pairs of said second laser sources in a tree-like pattern until all of said laser sources are coupled into a second common output; said second common output coupled to a second end of said double clad fiber amplifier pump core so as to propagate said pump core in a direction opposite the propagation of said first common output.
 80. The laser system of claim 79 further comprising at least one blocking filter incorporated in the optical coupling between the WDM coupling of all of said laser sources and each of said first and second common outputs to isolate said first and second sets of laser sources from one another.
 81. A fiber laser system comprising: a plurality of fiber lasers each having a given active dopant providing a different wavelength gain spectrum; a chirped fiber grating at an output of said fiber laser; said chirped grating allowing feedback over defined band of wavelengths so that said lasers operate in multiple wavelengths over their respective bands; and WDM combining means to receive said outputs from said fiber lasers.
 82. The fiber laser system of claim 81 wherein said chirp provides a degree of chirp that induces low noise operation.
 83. The fiber laser system of claim 81 wherein said chirp grating is narrower than the multiple wavelength gain spectrum of the given active dopant.
 84. A fiber laser system comprising: a fiber laser having a gain medium comprising a double clad fiber with its core doped with an active dopant; a single mode fiber coupled to receive the output of said fiber laser; a fiber grating contained in said single mode fiber; said grating providing feedback to stabilize the wavelength operation of said fiber laser.
 85. The fiber laser system of claim 84 wherein said coupling to said single mode fiber is via a fusion splice.
 86. The fiber laser system of claim 84 further comprising a quick disconnect coupler between said gain medium and said single mode fiber.
 87. The fiber laser system of claim 84 wherein a plurality of said fiber lasers have their outputs, via a respective fiber grating in a single mode fiber, WDM combined to produce a single common output.
 88. The fiber laser system of claim 84 wherein said fiber grating is chirped for reduced noise in system operation.
 89. The fiber laser system of claim 84 wherein a fiber grating is provided at opposite ends of said gain medium.
 90. The fiber laser system of claim 84 wherein a broad band reflector is provided at one end of said gain medium and said fiber grating is provide at the other end of said gain medium.
 91. A WDM laser system comprising: a plurality of fiber lasers, each of said fiber lasers doped with one or more active dopants that are homogeneously broaden providing sufficient gain for lasing action over a broad band of wavelengths; each of said fiber lasers induced to operate at a separate wavelength within the gain spectra of the respective active dopant(s) of each of said fiber lasers; and means for WDM combining said separate wavelength outputs.
 92. The WDM laser system of claim 91 wherein said active dopant is Yb.
 93. The WDM laser system of claim 91 wherein said active dopant is Yb in combination with at least; one other rare earth elements.
 94. The WDM laser system of claim 93 wherein said at least one other rare earth elements are Er or Nd.
 95. A fiber laser system comprising: a plurality of fiber lasers providing separate outputs at different wavelengths; and at least one of said output wavelengths being provided through Raman shifting in one of the fiber lasers.
 96. The fiber laser system of claim 95 further comprising: several fiber lasers all operating at different wavelengths; at least one of said lasers is a Raman laser; and means to WDM combine outputs from said fiber lasers.
 97. The fiber laser system of claim 95 wherein at least two of said fiber lasers are Raman lasers.
 98. The fiber laser system of claim 95 wherein at least some of said fiber lasers are coupled in series with at least one Raman laser.
 99. The fiber laser system of claim 95 wherein outputs of said fiber lasers are coupled in parallel.
 100. A gain medium system comprising: a plurality of gain media at least two doped with a different active dopant; said gain media producing an output of a different wavelength; the outputs from said gain media WDM combined and launched into a single mode fiber; and pumping means for excitation of each of said gain media.
 101. The gain medium system of claim 100 wherein said gain media are coupled in series end to end.
 102. The gain medium system of claim 100 wherein said gain media are in parallel with their outputs combined via one or more WDM devices in coupling pairs until a single output is achieved which is launched into said single mode fiber.
 103. The gain medium system of claim 100 wherein said different active dopants are selected from a group consisting of Er, Yb, Nd, Tm, Ho and Er:Yb.
 104. A laser gain media capable of producing gain over a broad band of wavelengths comprising: a plurality of gain producing fibers; a wavelength dispersing element for said gain producing fibers comprising a dispersive grating; an optical cavity formed by first reflective means in each of said gain producing fibers and a second reflective means comprising a broad band reflective mirror to provide feedback to said gain producing fibers respectively at different wavelengths, said first and second reflective means forming a lasing cavity in connection with each of said gain producing fibers; said gain producing fibers positioned so that their respective outputs impinge on said dispersive grating at different angular positions relative to a planar extent of said grating creating feedback to different wavelengths; said broad band reflective mirror producing a wavelength combined collinear output beam.
 105. The laser gain media of claim 104 wherein said gain producing fiber angular positions are set to provide a first order single reflected beam from said dispersive grating comprising a combination of the gain producing fiber outputs.
 106. The laser gain media of claim 105 wherein said first order single reflected beam is intercepted by said broad band reflective mirror.
 107. The laser gain media of claim 106 wherein said broad band reflective mirror is substantially transmissive of said first order single reflected beam providing a single beam output of high power for delivery to a beam application.
 108. The laser gain media of claim 107 wherein said application is marking, printing, material processing or heat treatment.
 109. A laser system comprising: a plurality of gain sources each providing a light output; means for pumping said gain sources a wavelength division multiplexer (WDM) device coupled to said gain sources to receive and combine the light outputs from each of said gain sources into a common output; and external feedback means coupled to receive the light output of at least some of said gain sources, said feedback means being broadband to permit lasing at multiple wavelengths by said gain sources.
 110. The fiber laser system of claim 109 wherein said laser sources are comprise of either semiconductor lasers or fiber lasers.
 111. The fiber laser system of claim 109 wherein said WDM device and said fiber grating are comprised of single mode fiber.
 112. The fiber laser system of claim 109 wherein said WDM device is a fused coupler.
 113. The fiber laser system of claim 109 wherein said fiber grating is a periodic grating or a chirped grating.
 114. The fiber laser system of claim 109 wherein said laser sources are double clad fiber lasers.
 115. The laser system of claim 109 wherein said feedback means is a fiber grating.
 116. The laser system of claim 109 wherein said gain source is a fiber gain source.
 117. The laser system of claim 109 wherein said grating is chirped.
 118. A laser system comprising: a plurality of laser sources each providing a light output; a wavelength division multiplexer (WDM) device coupled to said laser sources to receive and combine the light outputs from each of said laser sources into a common output; said system characterized by multimodal means coupled to the light output of at least some of said laser sources provide feedback causing them to operate with multimodes.
 119. The laser system of claim 118 said multimodal means comprises feedback reflection means coupled to receive the light output of at least some of said laser sources, said feedback reflection means having a broad reflection band to cause said laser sources to operate with multimodes.
 120. The laser system of claim 118 wherein said multimodal means comprises feedback reflection means coupled to receive the light output of at least some of said laser sources after being combined via said WDM device, said feedback reflection means having a broad reflection band to cause said laser sources to operate with multimodes.
 121. The laser system of claim 120 wherein said feedback reflection means comprises at least one fiber grating.
 122. The laser system of claim 121 wherein said fiber grating is a broadband grating.
 123. The laser system of claim 120 wherein said feedback reflection means comprises a plurality of fiber gratings, one for each of said sources.
 124. The fiber laser system of claim 118 wherein said laser sources comprise semiconductor lasers, single mode fiber lasers or double clad fiber lasers.
 125. The laser system of claim 118 wherein said multimodal means comprises a plurality of laser diodes, and feedback reflection means at said common output to cause said laser diodes to operate in multiple modes.
 126. The fiber laser system of claim 122 wherein said feedback reflection means comprises a broadband grating at said common output for coupling back a portion of the laser diode light outputs to said laser diodes to cause said laser diodes to have multimode operation.
 127. A laser system comprising: a plurality of laser sources each providing a light output; a wavelength division multiplexer (WDM) device coupled to said laser sources to receive and combine the light outputs from each of said laser sources into a common output; said system characterized by multimodal means comprising a plurality of multimode laser diodes with their light outputs coupled to said common output.
 128. The fiber laser system of claim 127 wherein the light outputs of said multimode laser diodes are coupled via multimode fibers to said common source.
 129. The fiber laser system of claim 128 wherein said common output comprises double clad fiber gain medium.
 130. The fiber laser system of claim 128 wherein said common output comprises double clad fiber gain medium.
 131. The fiber laser system of claim 130 wherein said double clad fiber gain medium comprises at least one fiber amplifier or at least one fiber laser.
 132. The fiber laser system of claim 131 wherein said fiber grating has a broad bandwidth so that if the polarization or wavelength response shifts due to said WDM device, said laser source will automatically seek optimal polarization and wavelength operation within the bandwidth of said fiber grating thereby leading to enhance operating stabilization.
 133. The fiber laser system of claim 131 wherein said WDM device is a fused coupler.
 134. The fiber laser system of claim 131 wherein said multimodal means comprises a fiber grating of a continuous or chirped period.
 135. The fiber laser system of claim 118 wherein said multimodal means comprises a plurality of laser diodes with their light outputs coupled via said WDM device to said common output comprising a fiber, and a plurality of fiber gratings in said fiber one for each of said laser diodes, each of said fiber gratings having a broad bandwidth so that if the polarization or wavelength response shifts due to said WDM device, said laser source will automatically seek optimal polarization and wavelength operation within the bandwidth of said fiber grating thereby leading to enhance operating stabilization. 